Field of the Invention
5-fluoro-1H-pyrazoles, in particular 5-Fluoro-1-methyl-3-pentafluoroethyl-4-trifluoromethyl-1H-pyrazole, are important building blocks for the preparation of crop protection chemicals, as those described in WO 2010051926.
Description of Related Art
It is known that 5-fluoro-1-methyl-3-pentafluoroethyl-4-trifluoromethyl-1H-pyrazole can be prepared by the treatment of the dimer of hexafluoropropene with water free N,N-dimethylhydrazine in diethyl ether at −50° C. followed by heating of the intermediate at 120° C., I. L. Knunyants et al. Izv. Akad. Nauk SSSR, (1990) 2583-2589.

However, this two step transformation requires low temperatures for the first step and results in the formation of CH3F during the thermal elimination in the second step, making this process expensive, environmentally unfriendly, and particularly difficult for industrialization.
Starting from perfluoro-2-methyl-2-penten and phenylhydrazine, in the presence of triethylamine at −50° C. 1-Phenylpyrazole has been shown to be obtainable in 90% yield (SU 1456419). Furin et al. J. Fluor. Chem. 98(1999) 29 reported that the reaction of perfluoro-2-methyl-2-pentene with phenylhydrazine in CH3CN gave a mixture of isomeric pyrazoles 3 and 4 in a ratio 4:1.

Although, commercially available at low cost (especially in the form of their water solutions) the use of monoalkylhydrazines for the regioselective synthesis of the said pyrazoles is not known from the prior art. The problem to be solved by this invention was to identify a simple and selective process for preparing 5-fluoro-1H-pyrazoles from available fluoroalkenes and mono-substituted hydrazines, which should in particular be amenable for an industrial scale process. As an additional advantage, this process should have a favorable profile with respect to safety and production of unwanted waste material.